I. Field of the Invention
The present invention relates to high radio frequency transmission lines.
II. Description of Relevant Art
It is well known that single electrical conductors, such as wires, are inadequate for conducting high frequency signals, i.e. radio signals in the microwave or millimeter wave range. Such high frequency signals, hereinafter collectively referred to as microwaves, are used for many different types of applications, such as radar and line of sight radio communication.
One previously known method of conveying microwave radio communications is to create a microwave transmission line. These previously known microwave transmission lines typically comprise a planar dielectric substrate having spaced apart upper and lower sides. The spacing between the upper and lower sides, i.e. the thickness, of the dielectric substrate will vary depending upon the operational radio frequency of the transmission line and the dielectric constant of the substrate.
In order to convey the microwave signal between two points on the substrate, a highly electrically conductive microstrip extends between the two points on the substrate. The microstrip is typically constructed of a metal, such as gold, silver, or copper, and extends between the two points on the substrate. The thickness of the microstrip typically is greater than five skin depths corresponding to the operating frequency of the transmission line and in which one skin depth is the distance penetrating into a signal conductor at which the alternating current density has exponentially decayed to 1/e of the value at the surface of the signal conductor that faces ground. As such, the microstrip is optically opaque. However, the width of the microstrip as it extends between the two points on the dielectric substrate is typically very narrow.
In order to complete the transmission line, a conductive ground plane is formed on the opposite side of the dielectric substrate. This conductive ground plane may also be constructed of a highly electrically conductive material, such as gold, silver, or copper, and, for greatest efficiency, preferably has a skin depth of greater than five skin depths corresponding to the operating frequency of the transmission line.
While these previously known microstrip transmission lines have proven effective in conveying microwave signals between different points in the microwave circuit, the previously known practice of forming the ground plane from a relatively thick conductive material, i.e. greater than five skin depths, is that the resulting structure is necessarily opaque to visible light. As such, the overall microstrip transmission line cannot be used for applications where transparency to visible light would be desirable. Such situations would include, for example, covert microwave antennas and other communication components.